3-(aminomethyl)-3,5,5-trimethylcyclohexyl amine (referred to herein as "isophorone diamine"), is used as a starting material for the preparation of isophorone diisocyanate, as an amino component for polyamides and as hardener for epoxide resins.
Isophorone diamine (IPDA) has been obtained by the reductive amination of 1,3,3-trimethyl-5-oxo-cyclohexane carbonitrile, (referred to herein as "isophorone nitrile" and as "IPN"), in the presence of ammonia and conventional hydrogenation catalysts. The isophorone nitrile used as starting material can be obtained by reaction of hydrogen cyanide and isophorone--cf. published German patent application P 39 42 371.9.
According to the method of German patent 12 29 078, ammonia and IPN are added in a molar ratio of 10-30 to 1 in order to obtain IPDA. However, in addition to the desired IPDA, a rather large amount of byproducts are produced, such as, in particular, 3-(aminomethyl)-3,5,5-trimethylcyclohexanol (=isophorone amino alcohol (IPAA)), 1,3,3-trimethyl-6-aza-bicyclo-3,2,1-octane and dihydroisophoryl amine. By way of example, a yield of up to 81.4% IPDA is disclosed but further purity data is absent. According to various sources, this yield has proved to be non-reproducible.
An attempt has been made to increase the yield of IPDA and to minimize the formation of IPAA in the process of German patent 12 29 078. According to the disclosure of published German patent application DE-OS 30 11 656, the method was changed by, in a first stage, converting IPN, without catalyst but with excess ammonia, into 1,3,3-trimethyl-5-imino-cyclohexane-carbonitrile. The latter was hydrogenated in a second stage to IPDA. A considerable excess of ammonia had to be used in the second stage. This mode of operation makes it necessary to use a complicated pressure distillation to recover and recycle the ammonia. According to the example given, a reaction yield of only 83.7% was achieved in the method of DE-OS 30 11 656, in spite of a ratio of approximately 5 kg ammonia per 1 kg IPN; no data is given about the ultimate yield of IPDA and its purity.
Published German patent application DE-OS 30 21 955 discloses that there was a further need to improve the methods of the foregoing disclosures. According to reference Example 1 of DE-OS 30 21 955, an IPDA yield of only 48% is achieved in spite of an IPN/NH.sub.3 volumetric ratio of 1 to 10 in the method of German patent 12 29 078. According to reference Examples 2 and 3 of DE-OS 30 21 955, carried out by the method of DE-OS 30 11 656, it was possible to obtain a yield of approximately 70% and 90%. However, the high yield required a long reaction time for the first stage and an IPN/NH.sub.3 volumetric ratio of 1 to 10 in the second stage. Thus, in addition to the disadvantage of the high excess of ammonia, there is also an economically significant reduction of the space-time yield.
DE-OS 30 21 955 disclosed that it was possible to reduce the long reaction time for the first stage--imine formation--in the method of DE-OS 30 11 656 by using an imine formation catalyst. However, it still was necessary to use a volumetric ratio of isophorone nitrile to ammonia of 1:10 to 20 in the second stage, and therewith an expensive system for pressure distillation, for the hydrogenation in the second stage. A further disadvantage is the fact that the reaction is complicated to carry out.
The method of published Japanese patent application JP-A 62-123154 involves the reductive amination of IPN for the preparation of IPDA, in which an attempt was made to reduce the required excess of ammonia and to eliminate the preliminary reduction of the carrier-supported catalyst. It should be possible, according to this method, upon using 1 to 20 times, preferably 5 to 10 times the molar amount of ammonia, relative to IPN, as well as Raney cobalt as catalyst, a pressure of 50 to 150 bars and a temperature of 50.degree. to 150.degree. C., to obtain IPDA in a high yield--the IPDA component in the reaction mixtures of the examples was approximately 83-89%, the IPAA component 4-6% (GC areal %). Considering the high amount of IPAA, which is difficult to separate, a not inconsiderable yield loss of IPDA must be dealt with in the purification by distillation. When the present inventors repeated the method of JP-A 123154, the statements of JP-A 62-123154 could not be confirmed. As is apparent from reference Examples 1 and 2 herein, IPN is hydrogenated only to an insufficient extent, to IPDA, under the conditions described in JP-A 62-123154.
It is clear from the state of the art described above that the desired isophorone diamine (IPDA) can only be obtained in good yield, with limited unavoidable accumulation of isophorone amino alcohol (IPAA), by the reductive amination of isophorone nitrile with ammonia and hydrogen via the intermediate product 1,3,3-trimethyl-5-imino-cyclohexanecarbonitrile, if ammonia was used in a very great excess. This excess of ammonia necessitated a complicated system, with apparatus for pressure distillation and recovery of the imine formation catalyst.